Fluorescence Resonance Energy Transfer (FRET) is a process whereby a first fluorescent dye (the “donor” dye) is excited, typically by illumination, and transfers its absorbed energy to a second dye (the “acceptor” dye) that has a longer wavelength and therefore lower energy emission. Where the second dye is fluorescent, energy transfer results in fluorescence emission at the wavelength of the second dye. However, where the second dye is nonfluorescent, the absorbed energy does not result in fluorescence emission, and the fluorescence of the initial donor dye is said to be “quenched”. Energy transfer can also be utilized to quench the emission of luminescent donors, including phosphorescent and chemiluminescent donors. When a luminescent emission is restored by preventing energy transfer, the luminescence is said to be “dequenched” or “unquenched”.
The use of a variety of dyes to quench fluorescence is known in the art. The application of this phenomenon to analyze biological systems is also well-detailed. FRET has been utilized to study DNA hybridization and amplification, the dynamics of protein folding, proteolytic degradation, and interactions between other biomolecules (Methods in Enzymology, Vol. 278). By far the most common donor-acceptor dye pair utilized for these applications is dabcyl (the quenching dye) and EDANS (the fluorophore) (as discussed in The FRET Probes, AnaSpec, 2004). Selected examples of biological applications of FRET can be found in the following references, among others:    (1) Holskin, B. P.; Bukhtiyarova, M.; Dunn, B. M.; Baur, P.; Dechastonay, J.; Pennington, M. W. Anal Biochem 1995, 227,148-155.    (2) Beekman, B.; Drijfhout, J. W.; Bloemhoff, W.; Ronday, H. K.; Tak, P. P.; te Koppele, J. M. FEBS Lett 1996, 390, 221-225.    (3) Pennington, M. W.; Thomberry, N. A. Peptide Research 1994, 7, 72-76.    (4) Wang, Q. M.; Johnson, R. B.; Cohen, J. D.; Voy, G. T.; Richardson, J. M.; Jungheim, L. N. Antivir Chem Chemother 1997, 8, 303-310.    (5) Gulnik, S. V.; Suvorov, L. I.; Majer, P.; Collins, J.; Kane, B. P.; Johnson, D. G.; Erickson, J. W. FEBS Lett 1997, 413, 379-384.    (6) Beekman, B.; van El, B.; Drijfhout, J. W.; Ronday, H. K.; TeKoppele, J. M. FEBS Lett 1997, 418, 305-309.    (7) Beebe, K. D.; Pei, D. Anal Biochem 1998, 263, 51-56.
Despite the widespread use of the dabcyl-EDANS energy transfer pair, this technology possesses a number of shortcomings. For most applications, the use of low wavelength excitation is not optimal due to the autofluorescence exhibited by most cellular systems. Ultraviolet light can also cause DNA cross-linking in some systems. In addition, if low wavelength excitation is used in a drug screening assay, many drugs, potential drugs, and biologically active proteins have very strong absorptions in the low wavelength region. Both dabcyl and EDANS have low extinction coefficients, resulting in assays that are comparatively insensitive.
In order to avoid the difficulties associated with the use of ultraviolet excitation, the absorption of the energy acceptor should be closely aligned with the visible light fluorophore used. The compounds of the instant invention have been discovered to quench the fluorescence of a large variety of dyes, including dyes that are excited in the ultraviolet, but also including fluoresceins, rhodamines, and even longer wavelength fluorophores such as Cy5 and allophycocyanin. In addition, the compounds of the invention have significantly larger extinction coefficients than the quenching compounds that are typically currently used in energy transfer assays.
The compounds of the instant invention represent a new and highly useful class non-fluorescent energy acceptors, including chemically reactive versions, and the conjugates prepared therefrom.